Postfix no mails are send out

I have a huge problem at the moment. My Debian 4.0 server has postfix/2.3.8 installed and properly running. Now, whenever I try to send mail using for instance a "lost password" form on a website that is hosted on that server the mail is not reaching the recipient but seems to stay in the queue.

Does this happen only with this recipient, or with all recipients (also recipients on other servers)? Because this looks like a problem on the receiving mail server to me, not a problem with your mail server.